This invention relates to microphone systems and, more particularly, to a direction finder employing microphones.
The availability of powerful, low-cost digital signal processors (DSPs) and programmable adaptive algorithms are increasingly allowing communications terminals to adapt to their environmental, user and network variations. Directional microphones, by their nature, can help mitigate the corrupting influence of room noise and reverberation on the performance of speakerphone systems. However, if narrow audio polar directivity patterns, i.e., directional beams, are to be steered in a full room coverage situation, then the talker""s locationxe2x80x94often rapidly changingxe2x80x94must be known. Another need for a xe2x80x9ctalker direction finderxe2x80x9d is in a multimedia communication or security product where a camera or display are directed. Yet another area of application for a talker direction finder might be to allow the near-end on a teleconference to identify which far-end participant is associated with the voice signal being received. In order to realize these applications, the talker (sound) direction finder would have to follow a rapidly moving talker (acoustic source), or switch to a new talker (acoustic source) readily and accurately, with full 360xc2x0 coverage.
One known direction finder arrangement is described in a thesis authored by D. M. Etter entitled xe2x80x9cDigital Signal Processing With Adaptive Delay Elementsxe2x80x9d, University of New Mexico, PhD. Thesis, 1979, which uses an adaptive, minimization technique to realize the audio polar directivity pattern. This arrangement requires, for a desired directional resolution, increased processing power as the microphone elements are spaced closer together. Alternatively, large spacing between the microphone elements is not physically advantageous in many applications because it limits bandwidth and requires talkers to stay farther from the microphone elements in order to retain accuracy. In either case, resolution is greatest in a direction perpendicular to a line between microphone elements and is therefore not uniform. If the directional range of this arrangement is to be extended from 180xc2x0 to 360xc2x0, two such arrangements are required. Additionally, the Etter arrangement requires phase information to be retained which would prohibit utilizing such techniques as a noise guard depending on long-term amplitude windowing or the like.
Another known arrangement is disclosed in U.S. Pat. No. 4,131,760 issued to Christensen and Coker on Dec. 26, 1978. The Christensen and Coker arrangement performs very well in many applications, particularly for large distances up to 50 feet away from the microphone elements. They describe 2.5 feet as a reasonable spacing between microphone elements to achieve a desirable resolution. Again, this relatively large spacing is to large for many applications, and leads to restrictions on how close a talker could approach the microphone elements without compromising accuracy. Greater amounts of signal processing could be used to circumvent these limitations. Again, the directional resolution of this arrangement is not uniform, and two such arrangements are required to realize 360xc2x0 coverage.
Problems and limitations with prior direction finder arrangements are overcome by employing a plurality of transducers to derive a plurality of predetermined polar directivity patterns each of which has a predetermined spatial orientation and pointing in a predetermined fixed direction relative to each of the other polar directivity patterns. The polar directivity patterns detect a plurality of amplitude values of a propagating wave approaching at different angles relative to the plurality of spatially oriented polar directivity patterns. Then, the detected wave amplitude values are processed to determine an estimate of a direction toward the source of the arriving wave. More specifically, the detected amplitude values are processed to obtain an estimate of the directional orientation of a hypothetical polar directivity pattern pointing toward the source of the arriving wave.
A technical advantage of the invention is that low cost, small sized omni directional microphones can be employed in forming the polar directivity patterns and that the microphones may be placed very close to one another.